A plum  is a stone fruittree in the genus Prunus, subgenus Prunus. The subgenus is distinguished from other subgenera (peaches, cherries, bird cherries, etc) in the shoots having a terminal bud and the side buds solitary (not clustered), the flowers being grouped 1-5 together on short stems, and the fruit having a groove running down one side, and a smooth stone. The fruit may have a dusty-white coating that is easily rubbed off -- this is an epicuticular wax coating on mature plum fruit gives them a glaucous appearance.